1. Field of the Invention
This invention relates to conversion between analog and digital signals, that is conversion from analog signals to digital signals and vice versa.
2. Description of the Prior Art
Analog to digital (A/D) and digital to analog (D/A) signal conversion are both subject to certain limitations in that either conversion process tends to introduce distortion into the signal being converted. For example, harmonic distortion can be introduced during conversion, in which one or more harmonics (namely frequencies which are multiples of a fundamental frequency component in the signal) appear in the converted signal. This can typically arise as a result of linearity errors in the conversion process, and results in undesired harmonics being introduced into the converted signal. In many applications, such as digital audio signal processing, this source of distortion leads to significant degradation of the signal being processed.
Conversion from analog to digital signals or vice versa is also subject to limitations in resolution. Any conversion device is capable of operating at a certain maximum resolution, and this sets a limit on the data resolution available in the converted signal.
International (PCT) Patent Application Publication No. WO 87/06079 discloses a technique of conversion between analog and digital signals in which a random signal of one (either analog or digital) form is added to the input signal having the same form. The summed signal is converted to the other (either digital or analog) form, and the random signal is also converted to that other form. The converted random signal is then subtracted from the converted summed signal to provide a representation of the input signal in converted form. This technique provides some reduction in the harmonic distortion caused by the analog-to-digital (or vice versa) conversion since the linearity errors of the conversion are randomized. However, the effectiveness of the technique relies on the addition of the random signal in one form being accurately matched by the subtraction of the random signal in the other form, so that the random signal may be effectively eliminated from the converted output signal. In practice, such accurate matching is difficult to achieve.